High resolution FTIR and diode laser supersonic jet Spectroscopy of the N=2 HF stretching polyad in (HF)2 and (HFDF): Hydrogen bond switching and predissociation dynamics

We report Fourier transform infrared (FTIR) and high resolution diode laser spectra (similar to 1 MHz instrumental bandwidth) obtained in cooled absorption cells as well as in a supersonic jet expansion for the N = 2 polyad region of the HF-stretching vibrations of (HF)(2), HFDF and DFHF. Three vibrational transitions have been observed for (HF)(2) and two for both monodeuterated isotoporners. For (HF)(2) we have identified and analysed the observed transitions of the polyad member 2(2) of the type Delta K-a = 0 and Delta K-a = +/- 1 up to rotational sublevel K-a = 3. Band centers as well as rotational constants of all four K-a states have been determined. The tunneling splittings due to hydrogen bond switching for these four K-a states have been investigated, with the K-a = 0 up to K-a = 2 sublevels having tunneling symmetry Gamma(vt) = A(+) for the lower tunneling states, and switching periods ranging from 158 ps for K-a = 0 to 1.35 ns for K-a = 2. A tunneling level inversion is found at K-a = 3, leading to a symmetry Gamma(vt) = B+ for the lower tunneling state of this (K)a-sublevel. The vibrational assignment of the measured spectra of (HF)(2) was established by comparison with the monodeuterated isotoporners HFDF and DFHF. For HFDF we have identified and analysed five subbands between 7600cm(-1) and 7730cm(-1). We have determined the spectroscopic constants of the rotational levels K-a = 0 and K-a = 1 for the vibrationally excited state and of the levels of K-a = 1 and K-a = 2 of the ground state, the latter from combination differences. From the measurements in a supersonic jet expansion we determined the predissociation line width of the N = 2(2), K-a = 1 to be about 120 MHz for the Gamma(vt) = A(+) tunneling state of (HF)(2) and about 90 MHz for Gamma(vt) = B+. For the K-a = 0 level of N = 2(2) we obtained predissociation line widths ranging around 100 MHz, similar to those of the K-a = 1 level. In the case of HFDF, the predissociation line width of K-a = 1 is about 80MHz. Predissociation lifetimes for these levels with the unbonded HF stretching excited thus are in the range of about 1 to 2 ns. The predissociation width in the N = 2(1) level is uncertain by about a factor three with 1g(Delta v/MHz) = (3 +/- 0.5) and in N = 2(3) it is about 600 MHz corresponding to rounded lifetimes of 0.1 ns and 0.3 ns when the bonded HF stretching is excited thereby demonstrating strongly mode selective predissociation rates in the N = 2 polyad. Under thermal equilibrium conditions we derived the pressure broadening coefficient for (HF)(2) (gamma = (6 +/- 1) x 10(-4)cm(-1)/mbar in the wavenumber range between 7713 cm(-1) and 7721 cm(-1) for total gas pressures between 10 and 60 mbar, all values as full widths half maximum). For absolute frequency calibrations we have remeasured the first overtone transitions of the monomer HF with much improved precision between P(5) (7515.80151 cm(-1)) and R(7) (7966.22188 cm(-1)).